Switch-mode power supplies (SMPS) used in consumer electronics, portable applications, and computers, are required to meet stringent voltage regulation requirements using a cost-effective implementation occupying a small volume. The regulation is usually achieved with on-chip integrated controllers which typically utilize voltage mode pulse-width modulation (PWM).
When a load change occurs, it is the task of the supply regulator to keep the supply voltage within a well-defined range in order to prevent overstress or performance degradation.
A fast reaction to such load changes is required to prevent too much deviation from the nominal supply-value. The traditional voltage mode PID control is not very suited for this, whether it is implemented in an analogue or digital way, mainly because the regulation parameter, usually the duty-cycle of the PWM control signal, is only updated once in a switching cycle. Thus, even with large gain-settings, it can take up to a switching period before a reaction is due.
A fast load change, in either direction, is often referred to as “transient”. Methods to recover quickly from such transients, called “transient improvement”, typically aim both to minimize the over- or under-shoot as well as to shorten the recovery time. Recovery time is defined as the time needed from the start of the transient to the point where the output voltage is back within the required steady-state regulation band.
Many methods of transient improvement have been proposed, such as Active Transient Response by Primarion and Non Linear Response by Zilker, These have recently gained acceptance due to the availability, and commercially affordability, of digital techniques.
Such methods try to react as soon as possible to a load transient and act accordingly.
Usually, an error signal is obtained and processed for analysis before the decision is made a real transient occurred. During this processing, however, valuable time is wasted, leading to an already large deviation of the output voltage from its reference value. The sooner corrective action can be taken, i.e. starting to transfer maximum possible energy to the output in the case of a light-to-heavy load change, or, stopping immediately any energy transfer in the case of a heavy-to-light load change, the smaller the resulting under- or over-shoot, respectively, will be.
Thus, for a good transient response, it is important to have a transient detection as fast as possible, whilst also being reliable and distinctive.
When corrective action has been taken (either maximum energy transfer or none at all), the output voltage (Vout) will eventually reach its maximum deviation (called the valley or peak, respectively). For many improvement methods, it is important to precisely determine the moment the valley or peak occurs. In other words, it is desired to accurately determine the moment when dVout/dt=0. Any substantial deviation or error in such detection will render the response non-ideal. It should be noted here that such a peak or valley occurs in a buck convertor (a SMPS which is a step-down DC to DC converter) on the moment when the current through the inductor becomes equal to the load current drawn from the convertor output.
A complication here is the fact that the measured Vout is not the real voltage on the intrinsic output capacitor Cout, due to the parasitic series resistance ESR (Resr) of the capacitor. It can be easily derived the voltage on the intrinsic capacitor will lag by a time tesr=Resr×Cout, for which a correction can be made when determining the peak/valley by observing Vout.
Shortly after a load change occurs, a second change may occur, in the same or opposite direction. This is commonly referred to as “multi load step”. Many methods of transient improvement lack the possibility to detect such consecutive transients, while still handling the first one, possibly leading to unwanted large output voltage excursions.
Among the most commonly used solutions in digital implementations for obtaining the error signal, being the difference between the voltage set-point Vref and the output voltage Vout, the following methods are known:
1) A low resolution Flash ADC, converting only the deviation of Vout. This is small and efficient, but suffers from a limited semi-fixed conversion range (which only changes when the set-point Vref changes).
2) A very fast high-resolution ADC, covering the total Vout range. This has the drawback of requiring a large silicon area and is power hungry.
3) A so-called Track-and-Detect ADC which is small and effective in transient detection, but still requires error tracking in order to determine the peak or valley.
The most common way of digitising the error signal is by using a differential amplifier followed by a fast Flash ADC, as depicted in FIG. 1.
The differential amplifier 10 produces the differential voltage of the two terminals Vout—p and Vout—n, between which the generated output voltage Vout is regulated. Vout is connected to all positive inputs ‘in’ of first to Nth comparators 12. The reference inputs ‘ref’ of the comparators 12 each have a different reference level, both positive and negative, and not necessarily equidistant, with respect to the reference voltage Vref, in essence the set-point. For each comparator whose input ‘in’ sees a higher voltage than its reference input ‘ref’, the output will be high. Thus, the generated output of the Flash ADC will behave like a thermometer code, which may be subsequently encoded into a form of error code.
The number N of comparators 12, in combination with the reference levels, determines the accuracy and the useful range of the error measurement. For an accurate peak-/valley-detection a small difference between the reference levels is preferred. If the peak or valley always occurs in the same region, this solution may be adequate, but this is not usually the case and thus a large number of comparators is generally needed, making this known solution unsuitable for many applications.
Another way of addressing the accuracy issue is to apply a full-fledged, fast, high-resolution ADC (as shown in FIG. 2), to digitise the absolute value of Vout with the required number of bits. Such an ADC, however, usually consumes a large amount of current and occupies a relatively large area in silicon. Moreover, a transient can only be detected by processing the error signal, unless two extra comparators are added to perform this function, but with the same multi load step detection problem as in the previous solution.
A third option is the so-called Track-and-Detect ADC, as depicted in FIG. 3.
Here, two (static) comparators 30 are used for steady-state regulation and two other (dynamic) comparators 32 are used for transient- and peak-/valley-detection, thus requiring only a very small silicon area. However, this approach lacks the possibility to detect a transient faster than via error processing. It will be appreciated that this can be solved by adding two dynamic comparators, with their reference levels forming a somewhat larger window than the peak/valley detection window, to retain the transient detection at all times. Nonetheless, the peak-/valley-detection accuracy depends on the achievable detection window of the two dynamic comparators 32.
A smaller “virtual” window can be obtained by first amplifying the error signal (Vout−Vref), by means of a differential amplifier 34 (having a gain=1+R/R=2 in the arrangement of FIG. 3), before feeding the output (Vdiff) of the differential amplifier 34 to the dynamic comparator 32 inputs ‘in’. Here, a simple (analogue) way of filtering spikes and noise is provided by using a feedback capacitor 36.
Whenever Vdiff hits either side of the peak/valley detection window, defined by the reference levels of dynamic comparators 32, the logic gate 38 will close switch 40 until the voltage on hold capacitor 42 is within the detection window again, and so on and so forth, making the voltage on hold capacitor 42 effectively follow Vdiff. Thus, a peak or valley can always be detected by observing a change in the sign of the error signal, derived from the dynamic comparator 32 outputs.
As mentioned above, although the arrangement of FIG. 3 is effective in transient detection, it still requires error tracking in order to determine a peak or valley.